Many types of power equipment employ friction clutches between a drive mechanism (usually a gas or electric motor) and a driven mechanism (i.e., the functional part of the equipment). For example, friction clutches may be used in power trowels for smoothing concrete pads, the clutch being used to selectively engage and disengage the motor with the trowel blade to cause the blade to rotate or cease rotating, respectively. Similarly, friction clutches may be used in hydraulic pumps, stationary or trailer-mounted power units, portable sawmills, agricultural equipment, fans and blowers, and lawn and garden equipment.
In most of these cases, it is desirable to be able to control engagement of the clutch as the motor causes the driven mechanism to begin rotating from a stationary state. More importantly, it is often critical for the safety of an operator to be able to rapidly disengage the clutch to cease power input to the driven mechanism and allow the driven mechanism to stop rotating quickly. Further, it is preferable to have a clutch that is, by default, disengaged (i.e., it fails to a safe, disengaged state) to avoid the dangers that may be created should the drive mechanism be started while the clutch is engaged, which may cause the driven mechanism to start rotating unexpectedly.
For example, existing power trowels use a centrifugal clutch with a set throttle position. If the operator loses control of the trowel, or inadvertently releases the handle while the blade is engaged in smoothing concrete, the continued operation of the trowel may cause the handle to spin around and hit the operator with extreme force, causing severe injury or even death. Some power trowels include an inertia switch or operator presence switch to shut off the trowel should the operator lose control, but in many cases, trowels with such switches will continue to rotate, potentially striking the operator, when the operator loses control. Also, such switches can be, and often are, easily bypassed, so that if the operator loses control, the trowel will “helicopter” continuously until it runs out of gas or comes up a against a fixed object (e.g., a wall or a piece of heavy construction equipment) that retards or stops its motion.